Calibration and alignment of X-ray reflectometric systems

ABSTRACT

The present invention relates to the calibration and alignment of an X-ray reflectometry (“XRR”) system for measuring thin films. An aspect of the present invention describes a method for accurately determining C 0  for each sample placement and for finding the incident X-ray intensity corresponding to each pixel of a detector array and thus permitting an amplitude calibration of the reflectometer system. Another aspect of the present invention relates to a method for aligning an angle-resolved X-ray reflectometer that uses a focusing optic, which may preferably be a Johansson crystal. Another aspect of the present invention is to validate the focusing optic. Another aspect of the present invention relates to the alignment of the focusing optic with the X-ray source. Another aspect of the present invention concerns the correction of measurements errors caused by the tilt or slope of the sample. Yet another aspect of the present invention concerns the calibration of the vertical position of the sample.

FIELD OF THE INVENTION

X-ray reflectometry is a technique for measuring the thicknesses of thinfilms in semiconductor manufacturing and other applications. In order tomaximize accuracy with this technique, it is necessary to preciselycalibrate and align elements of the X-ray reflectometry system and thepresent invention relates to methods for achieving this.

BACKGROUND OF THE INVENTION

There is considerable need to accurately measure the thicknesses of thinfilms, particularly in the semiconductor manufacturing industry. Onemethod for making such measurements is an X-ray reflectometry technique(“XRR”) which relies on measuring the interference patterns of X-raysscattered from a thin film sample. With XRR the reflectivity of a sampleis measured at X-ray wavelengths over a range of angles. These anglestypically range from zero degrees, or grazing incidence along thesurface of the sample, to a few degrees. From the X-ray interferencepattern, properties of the sample such as material composition andthickness can be inferred.

In a recent development, simultaneous measurements of the samplereflectivity over a range of angles are accomplished by illuminating thesample with a focused beam and then detecting the reflected X-rays witha position sensitive detector such as a photodiode array.

XRR has several advantages over techniques using visible light. One suchadvantage is that XRR makes it possible to measure the thickness ofultra-thin films whose thicknesses are on the order of 30 angstroms orless. Visible light is not suitable for the study of such ultra-thinfilms using interference patterns because of its wavelength. However, anXRR system may preferably use radiation at wavelengths of about 1.5angstroms, which radiation creates suitable interference patterns evenwhen probing such ultra-thin films. In addition, XRR may suitably beused where the film is composed of a material that is opaque to light,such as a metal or metal compound. Finally, XRR may suitably be used tomeasure the density and thickness of films composed of materials thathave a low dielectric constant and a correspondingly low index ofrefraction, such as certain polymers, carbon fluoride compounds, andaerogels.

A preferred XRR technique is described in U.S. Pat. No. 5,619,548,issued Apr. 8, 1997, which is hereby incorporated by reference in itsentirety. FIG. 1 illustrates this preferred technique.

Referring to FIG. 1, the preferred X-ray scattering system includes anX-ray source 31 producing an X-ray bundle 33 that comprises a pluralityof X-rays shown as 35 a, 35 b, and 35 c. An X-ray reflector 37 is placedin the path of the X-ray bundle 33. The reflector 37 directs the X-raybundle 33 onto a test sample 39 held in a fixed position by a stage 45,and typically including a thin film layer 41 disposed on a substrate 43.Accordingly, a plurality of reflected X-rays, 57 a, 57 b, and 57 cconcurrently illuminate the thin film layer 41 of the test sample 39 atdifferent angles of incidence.

The X-ray reflector 37 is preferably a monochromator. The diffraction ofthe incident bundle of X-rays 33 within the single-crystal monochromatorallows only a narrow band of the incident wavelength spectrum to reachthe sample 39, such that the Brag condition is satisfied for this narrowband. As a result, the plurality of X-rays 57 a, 57 b, and 57 c, whichare directed onto the test sample 39, are also monochromatic. A detector47 is positioned to sense X-rays reflected from the test sample 39 andto produce signals corresponding to the intensities and angles ofincidence of the sensed X-rays. FIG. 2 depicts an example of a graph ofdata from the detector 47 showing a normalized measure of thereflectivity of the sample as a function of the angle of incidence tothe surface of the sample 39. A processor is connected to the detectorto receive signals produced by the detector in order to determinevarious properties of the structure of the thin film layer, includingthickness, density and smoothness.

In order to maximize the accuracy of the X-ray measurements, it isnecessary to precisely calibrate and align the XRR system. The presentinvention relates to techniques for doing this.

SUMMARY OF THE INVENTION

One object of the present invention relates to the calibration of thedetector 47. In order to properly interpret the raw data graphed in FIG.3, it is necessary to determine which pixel C₀ lies on the extendedplane of the sample 39. In addition it is necessary to find theintensity of the incident, unreflected X-ray corresponding to each pixelin order to be able to normalize the reflected X-ray intensity readingson a point-by-point basis. An aspect of the present invention describesa method for accurately determining C₀ for each sample placement and forfinding the incident X-ray intensity corresponding to each pixel andthus permitting an amplitude calibration of the reflectometer system.

Another object of the present invention relates to a method for aligningan angle-resolved X-ray reflectometer that uses a focusing optic, whichmay preferably be a Johansson crystal. In accordance with the presentinvention, the focal location may be determined based on a series ofmeasurements of the incident beam profile at several different positionsalong the X-ray optical path.

Another object of the present invention is to validate the focusingoptic. It is important that the focusing optic forms an X-ray beam ofuniform and predictable convergence, This is necessary in order toachieve an accurate one-to-one correspondence between the pixel locationon the detector and the angle of reflection of X-rays from the sample. Avalidation of the optics may be performed using a grid mask consistingof regularly spaced openings and opaque bars in order to observe theaccuracy of optic shaping.

Another object of the present invention relates to the alignment of thefocusing optic with the X-ray source. For example, in the case of anX-ray tube source, achieving the best angular resolution for thereflectometer requires that the line focus of the X-ray tube and thebend axis of the focusing optic be coaligned so as to be accuratelyparallel. A method for checking this coalignment is to place a fine wirebetween the X-ray source and the optic and observe the shadow of thewire in the beam profile formed by the optic.

Another object of the present invention concerns the correction ofmeasurements errors caused by the tilt or slope of the sample.

Yet another object of the present invention concerns the calibration ofthe vertical position of the sample. Changes in the sample height leadto shifts in the location of the reflected beam, so that the verticalsample position must be calibrated if an accurate measurement is to bemade.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a preferred X-ray reflectometry system.

FIG. 2 shows a normalized graph of sample X-ray reflectivity as afunction of the angle of incidence to the sample.

FIG. 3 shows a graph of raw data from a position sensitive detector inan XRR system giving signal strength as a function of pixel number.

FIG. 4 shows graphs of both the incident X-ray beam and the reflectedX-ray beam (dashed lines) as received by the position sensitive detectorin an XRR system.

FIG. 5 shows a graph where the measurements of the pixels sensing theincident X-ray beam have been inverted through a calculated pixel C₀ sothat the incident beam is “folded over” the reflected X-ray beam asmeasured by the position sensitive detector in an XRR system.

FIG. 6 shows graphs of the incident beam profile with a detector placedat three different “run-out” distances from the nominal focal locationof the X-ray optic.

FIG. 7 shows a plot of the locations of the upper and lower limbs of thebeam profile graphs shown in FIG. 6 as a function of “run-out” distance.

FIG. 8 shows a graph of both the original incident X-ray beam and theprofile of the incident X-ray beam after it has passed through a grid asmeasured by a detector.

FIG. 9 shows a graph of the incident X-ray beam profile where the beampath is partially blocked by a wire.

FIG. 10a shows the relationship of X-ray angle of incidence and detectorpixels in an XRR system when the sample is not tilted.

FIG. 10b shows the altered relationship of X-ray angle of incidence anddetector pixels in an XRR system when the sample is tilted.

FIG. 11 shows a system for detecting sample tilt.

FIG. 12 shows how changes in sample height alter the relationshipbetween X-ray angle of incidence and detector pixels in an XRR system.

FIG. 13 a system for detecting changes in the sample height.

FIG. 14 shows a method for finding the intensities of the unattentuatedX-rays at each angle of incidence in order to be able to properlynormalize the measurements of the X-ray detector.

DETAILED DESCRIPTION OF THE INVENTION

One aspect of the present invention relates to the calibration of thedetector 47. For convenience, this aspect of the present invention willbe described with respect to the preferred type of detector, amulti-channel photo-diode array, but the same technique could be appliedto other types of spatially sensitive detectors capable of resolving thereflected X-rays from the sample 39. The detector yields raw datashowing the intensity of the reflected X-rays as a function of thedetector channel (“pixel”) number, as shown graphically in FIG. 3. Inorder to properly interpret the raw data graphed in FIG. 3, it isnecessary to determine which pixel C₀ lies on the extended plane of thesample 39. In addition it is necessary to find the intensity of theincident, unreflected X-ray corresponding to each pixel in order to beable to normalize the reflected X-ray intensity readings on apoint-by-point basis. This calibration is necessary because thereflection efficiency of the sample for each angle of incidence isactually calculated as a ratio of reflected and incident signalamplitudes. This aspect of the present invention describes a method foraccurately determining C₀ for each sample placement and for finding theincident X-ray intensity corresponding to each pixel and thus permittingan amplitude calibration of the reflectometer system.

The relationship between the reflection angle θ and the pixel number Cis given by θ=arctangent (p (C−C₀)/D), where p is the pixel spacing(“pitch”) of the detector, D is the distance between the illuminatedpart of the sample and the detector, and C₀ is the pixel number at whichthe extended plane of the sample intercepts the detector. The parametersp and D are customarily known with sufficient accuracy from theconstruction details of the detector (for the pitch p) and thereflectometer (for the distance D). The value C₀, however, can vary fromone sample placement to another, and must be determined (or at leastverified) for each measurement.

In order to accomplish these angular and amplitude calibrations of thereflectometer, a detector is used that extends below the plane of thesample as well as above it. The use of such a detector 47′ is shown inFIG. 14, where the dashed lines indicate that the sample 39 has beenremoved from the X-ray pathway. By occasionally removing the sample fromthe X-ray pathway, the unattenuated incident beam can then be detectedon the lower portion of the detector. In this way the incident beampattern can be recorded and viewed side-by-side with the reflected beam,as shown in FIG. 4. C₀ is a point of symmetry that lies midway betweenthe leading edges of the incident and reflected patterns. An approximatelocation for C₀ is shown in FIG. 4. The reason this is so is that thereflection from the sample is nearly total for a range of very smallangles of incidence that are less than the critical angle. For thislimited range of angles, then, the incident and reflected patternsshould be symmetrical about C₀. This means that C₀ is represented by thepixel number about which the incident beam pattern can be “folded over”to overlap the reflected pattern. The folding is correct and C₀ isproperly identified when the leading edge of the reflected pattern inthe region of total reflection falls exactly over the correspondingleading edge of the incident profile.

C₀ can accordingly be located in the following manner. First, onelocates the peak of the reflected signal. Because the samplereflectivity drops off very quickly at angles greater than the criticalangle, the peak of the reflected profile lies close to the criticalangle and serves to identify an approximate upper bound for the regionof the reflected profile that is of interest. To avoid getting data frombeyond the critical angle, the upper limit for the region of interest ispreferably set to 90% of the peak of the reflected beam. Second, onelocates the point where the incident and reflected profile signalscross. This point is approximately the location of C₀ and serves toidentify an approximate lower bound for the region of the reflectedprofile that is of interest. Because data from very weak signals tendsto be bad and may be significantly corrupted by noise, the lower limitis preferably set to a signal level that is twice the level at which thetwo signals cross. Third, one chooses various signal levels in theregion of the reflected profile curve that is of interest, and drawshorizontal lines between the reflected and incident profile curves atthose signal levels. Fourth, one finds the midpoints of the horizontallines between the two profiles drawn in the previous step. Fifth, onecalculates the average center pixel C₀ from this set of midpoints afterexcluding outlying data points.

Once the point of symmetry C₀ has been determined, the incident beam canbe folded over the reflected beam by inverting the pixel arrayassociated with the incident profile through the point C₀. Theseoperations produce a graph such as that shown in FIG. 5. A normalizedreflectivity is then calculated by dividing the raw values in thereflected profile by the corresponding incident profile values on apoint-by-point basis. In this way the reflected profile data from thedetector may be properly interpreted.

Another aspect of the present invention relates to a method for aligningan angle-resolved X-ray reflectometer that uses a focusing optic, whichmay preferably be a Johansson crystal. It is necessary to determinewhere in space the focused image of the X-ray source region falls, inorder to establish the distance D between the focused image and thedetector. As discussed above, this parameter is used to match pixels toreflection angles via the reflection angle correlation: θ=arctangent (p(C−C_(o))/D). It is also desirable to know the focal location, becausethis is the preferred position of the front surface of the thin filmsample being examined by the reflectometer in order to minimize thatX-ray beam “footprint” or irradiated area on the surface of the sample.

In accordance with the present invention, the focal location may bedetermined based on a series of measurements of the incident beamprofile at several different positions along the X-ray optical path. Ameasurement series of this sort is shown graphically in FIG. 6, in whichthe incident beam profile is detected at several values of “run-out”distance from the nominal focal location. (Points “A”, “B”, and “C” ofFIG. 14 illustrate this methodology.) In FIG. 6 the width of the profiledecreases as the detector location approaches the nominal focallocation. The actual focal location is the position in space at whichthe profile width extrapolates to zero. In FIG. 7 the locations of theshoulders of the incident beam profiles are plotted and denoted as thelower and upper limbs. The focal location is then the point at which thelower and upper limbs converge.

In order to find this convergence point, the incident beam edge datacollected at the various run-out locations may be fitted by linearregression techniques to obtain a pair of algebraic relations describingthe lower and upper edge locations as a function of run-out distance.The actual focal location is then determined by solving algebraicallyfor the intersection point of the two traces. In the case shown in FIG.6, we find that the actual focal location is about 15 mm closer to thedetector than the nominal value. The edge trace data also shows theminimum and maximum angles of incidence contained in the converging fanof X-rays that expose the sample. These angular values are reflected inthe slopes of the traces.

Another aspect of the present invention relates to the validation of thefocusing optic. It is important that the focusing optic forms an X-raybeam of uniform and predictable convergence. This is necessary in orderto achieve an accurate one-to-one correspondence between the pixellocation on the detector and the angle of reflection of X-rays from thesample. A validation of the optics may be performed using a grid maskconsisting of regularly spaced openings and opaque bars in order toobserve the accuracy of optic shaping.

In order to accomplish this, the grid mask is placed across the X-raybeam path between the X-ray source and the optic, and the shadow patternformed by the mask is detected at a position downstream from the optic.Data of this sort, formed by a mask having regularly spaced 75 μm-wideopenings and bars, is shown in FIG. 8. Deviations from these locationsare produced by distortions of the optic from its intended figure. Ifthe optic is correctly formed, the features of the observed gridpattern, its minima and maxima, should fall in predictable locationsbased on the opening and bar spacings of the grid. In this way thefocusing optic may be validated.

Another aspect of the present invention relates to the alignment of thefocusing optic with the X-ray source. For example, in the case of anX-ray tube source, achieving the best angular resolution for thereflectometer requires that the line focus of the X-ray tube and thebend axis of the focusing optic be coaligned so as to be accuratelyparallel. A method for checking this coalignment is to place a fine wirebetween the X-ray source and the optic and observe the shadow of thewire in the beam profile formed by the optic. A pattern of this sort isshown in FIG. 9. One can use a pinhole photograph of the X-ray source todetermine the orientation of its line focus and to prealign the wire tothat orientation. The width of the wire's shadow is then a measure ofthe tilt misalignment of the optic with respect to the tube's linefocus. An accurate coalignment can then be obtained by manipulating theoptic's tilt so as to minimize the width of the shadow.

Another aspect of the present invention concerns the correction ofmeasurements errors caused by the tilt or slope of the sample. The slopeof the sample is a critical parameter. Small variations in the sampleflatness or the mechanical slope in the supporting stage can lead tovariations in this plane as shown in FIGS. 10a and 10 b. As thesefigures show, changes in the sample tilt change the direction of thereflected beam. In particular, tilts along the direction of the beamtravel (“pitch”) cause the beam to shift up or down on the detector.Essentially, the rays at each incident angle are redirected to differentpixels. If such tilt shifts are not accounted for, the calculatedangular reflectivity will be wrong.

Errors of less than 0.005° in the sample tilt can change the filmthickness calculations by as much as a few angstrom, which is an amountthat is greater than the inherent sensitivity of the X-ray reflectometermeasurement. In other words, sample tilts in this range can be the majorsource of measurement error. To limit the stage tilt to a tolerablelevel of, say, 0.001° requires less than 1.8 microns of vertical errorover a 4″ radius (a typical radius for a silicon wafer).

Tilt in a direction perpendicular to the beam direction (“roll”) willalso alter the direction of the reflected beam. In this case, thepredominant effect is a side-to-side shift of the beam on the detector.Since the beam strikes the same pixel range, however, the relationshipbetween pixel and incidence angle is preserved. Thus, roll is a lesscritical phenomenon than pitch. Still, irregularities in the beam shapecould give rise to measurement errors if the roll were sufficientlysevere to alter the intensity of the detected portion of the beam.

Since variations in the tilt of the sample surface at the milli-degreelevel are almost inevitable, it is necessary to have some means ofdealing with tilt—especially pitch. FIG. 11 illustrates a preferred tiltdetection scheme.

As depicted in FIG. 11, a laser beam is split by a beam splitter 60oriented at a 45° angle. Part of the beam continues toward the sample 62and is focused to a point by a lens 64. A reflected beam reflects fromthe sample 62 and passes back up through the same focusing optics. Partof the reflected beam is reflected by the beam splitter 60 to a positionsensitive detector 66 which may preferably be a quad cell photodiodedetector. A change in the sample orientation (shown by the dotted lines)causes the return beam to shift. This displacement can be measuredquantitatively with the quad cell photodiode 66. The advantages of thismethod include its non-contact nature, high accuracy (determined by thefocal length of the lens 64 and the sensitivity of the quad cell 66),and the fact that it can be conducted concurrently with XRR measurementsbecause the quad cell detector 66 views the sample from above. With thismethod, tilts well below 0.001° may be measured.

Preferably the tilt detector described above may be calibrated usingsamples whose tilt is known. Alternatively, an approximate mathematicalrelationship between the readings of the quad cell detector and thesample tilt may be used to interpret the data. For this purpose it maybe assumed that the light coming into the lens 64 of FIG. 11 iscollimated and has uniform intensity over the illuminated circle on thelens (radius r), and that the lens aperture is larger than thisilluminated spot. If the lens 64 has a focal length f and the wafer istilted by a small angle d, then the reflected beam, 60, is shifted by adistance s on the quad cell detector 66, which distance s is given by:

s=2×f×d

(where d is measured in units of radians).

For purposes of illustration it is convenient to consider the case wherethe tilt is in the plane of FIG. 11 (i.e. the axis of tilt rotation isabout the normal to the paper), so that the reflected beam shiftsupwards on the quad cell, 66, as shown by the dashed lines.

The quad cell is composed of 4 quadrants which each produce an outputsignal proportional to the intensity on the quadrant: Q1, Q2, Q3, & Q4(quadrants 1&2 are on top; 3&4 are on bottom; and 1&4 are on the rightside). We can define the vertical tilt signal as

Ty=(Q 1+Q 2−Q 3−Q 4)/(Q 1+Q 2+Q 3+Q 4)=[(top−bottom)/sum]

(Here Ty means tilt about the vertical (y) axis to distinguish from tiltin orthogonal direction: Tx.)

When the quad cell is illuminated by a beam reflected from a non-tiltedsample, each quadrant should produce signals of the same strength q0. Inthis case, the vertical tilt signal is

Ty=(q 0+q 0−q 0−q 0)/(4×q 0)=0

For very small tilts that shift the beam upwards, the upper two quadsignals increase and the lower two decrease. If the spot on the quadcell shifts up a distance s, then the increase in the signal Q1 is

ΔQ 1=(4×q 0×s)/(π×r);

where π=3.14159 . . . =(8×f×d×q 0)/(π×r) (using the earlier equation fors).

In this case, the tilt signal becomes:

Ty=ΔQ 1/q 0=(8×f×d)/(π×r)

This equation can be reversed to solve for the tilt angle d as afunction of the measured tilt signal:

d=(Ty*π*r)/(8×f).

The same kind of analysis may be applied in the case of an orthogonaltilt, Tx, or the case where there is both a vertical and orthogonaltilt.

Once the sample tilt has been independently measured, it is necessary tocorrect for the tilt. Either of two methods of correction may be used.In one method, the tilt may be corrected for in analyzing the datareceived from the X-ray detector. Alternatively, the orientation of thestage on which the sample rests may be actively controlled in order toreduce the tilt.

If the tilt of the stage is actively controlled, the tilt sensor readingmay be used to purposely set the stage tilt to some non-zero angle. Thiscould be useful in studying films with particularly large criticalangles because tilting the stage would shift the incidence angle rangeto higher angles.

Yet another aspect of the present invention concerns the calibration ofthe vertical position of the sample. Changes in this sample height leadto shifts in the location of the reflected beam as shown in FIG. 12. Theshift of the reflected profile on the detector array is approximately2δ, where δ is the focus error, and d is the distance from the focus tothe detector. This results in an angular error of about δ/d, where d isthe distance from the focus to the detector. (This is so because thezero angle is interpreted to be the midpoint between the incident andreflected beams.) For systems designed to measure 8″ wafers, d could beas small as about 5″ which means that for every 10 microns of focuserror, an angular error of about 5 milli-degrees is introduced. Errorsof this magnitude may alter the thickness readings by several angstromsand constitute a major limitation on measurement accuracy.

The present invention provides a method of controlling this source oferror by quickly and accurately bring the XRR system into focus. Thismethod for autofocusing works by measuring the collimation of a beamreflected from the sample surface through a focusing lens as shown inFIG. 13. The assignee of the present invention has previously employed asimilar methodology for measuring both the tilt of the sample and thevertical position of the sample in the context of prior art light-basedbeam profile reflectometry and beam profile ellipsometry (see U.S. Pat.Nos. 5,042,951; 5,412,473; and PCT publication WO 92/08104. However, theinventors believe that they are the first to recognize that such amethodology could be used to solve the distinct problems characteristicto the proper calibration and operation of an X-ray reflectometrysystem.

As depicted in FIG. 13, a laser beam is split by a beam splitter 60oriented at a 45° angle. Part of the beam continues toward the sample 62and is focused to a point by a lens 64. A reflected beam reflects fromthe sample and passes back up through the same focusing optics. Part ofthe reflected beam is reflected by the beam splitter 60 and is focusedby a second focusing lens 68. This second lens 68 brings the reflectedbeam into focus near a spinning knife edge or “chopper” 70, located inthe focal plane of the lens 68. The location of this secondary focus is,in turn, dependent on the height of the sample. As the sample moves up,for instance, the secondary focus moves downstream. The direction andspeed of the chopper's shadow is detected by a position sensitivedetector 66 which may preferably be a quad cell photodiode detector. Ifthe chopper is not located at the focus, one side or the other of thequad cell will be darkened first by the shadow of the chopper, dependingon which side of the focus the chopper is located. By measuring themovement of the chopper's shadow, it is possible to calculate both theposition of the secondary focus relative to the position of the chopper70 and the sample height.

The precision of this method depends on the focal lengths and aperturesof the various components and increases with the magnification of thelenses used.

The autofocus signal produced by the above described detector scheme maybe defined as the timing difference Δt between the time when the top ofthe detector is shadowed and the time when the entire detector isshadowed. Δt can be positive or negative depending on whether the samplesurface is above or below the focus. The sensitivity of the system 9 canbe defined as the measured time gap for a given focus error:

S=Δt/Δz,

where Δz is the distance of the objective lens 64 from correct focus.(Like Δt, Δz may be positive or negative.)

If the objective lens 64, has a focal length f1, and second lens 68, hasa focal length of f2, and the beam incident on the first lens iscollimated and has a radius of r, then the sensitivity S of theautofocus detector to small focus errors is given by

S=2×r×f 2/(f 1×f 1).

From these equations it is possible to calculate Δz given knowledge ofΔt and the parameters r, f1, and f2. Given Δz, the distance of theobjective lens 64 from correct focus, an appropriate adjustment in therelative sample height may be made.

The scope of the present invention is meant to be that set forth in theclaims that follow and equivalents thereof, and is not limited to any ofthe specific embodiments described above.

What is claimed is:
 1. A method of measuring the characteristics of athin film layer on the surface of a sample comprising the steps of:generating a probe beam of X-rays; focusing said probe beam on thesurface of said sample such that various X-rays within the focused probebeam create a range of angles of incidence with respect to said surface;measuring the intensity of various X-rays as a function of positionwithin the probe beam as reflected with the positions of the X-rayswithin said reflected probe beam corresponding to specific angles ofincidence with respect to said surface; and separately measuring thetilt of said sample in order to either adjust said tilt or to correctthe X-ray measurements to account for said tilt.
 2. A method as recitedin claim 1 wherein said tilt is adjusted based on said tiltmeasurements.
 3. A method as recited in claim 1 wherein said X-raymeasurements are corrected based on said tilt measurements.
 4. A methodof measuring the characteristics of a thin film layer on the surface ofa sample including the steps of generating a probe beam of X-rays;focusing said probe beam on the surface of said sample such that variousX-rays within the focused probe beam create a range of angles ofincidence with respect to said surface; measuring the intensity ofvarious X-rays as a function of position within the probe beam asreflected with the positions of the X-rays within said reflected probebeam corresponding to specific angles of incidence with respect to saidsurface; and separately measuring the tilt of said sample in order toeither adjust said tilt or to correct the X-ray measurements to accountfor said tilt, said tilt measuring comprising the steps of: generating abeam of light; focusing said beam of light on the surface of saidsample; measuring the reflected beam of light with a position sensitivedetector; and determining changes in the tilt of said sample based onthe measurements made by the position sensitive detector.
 5. A method asrecited in claim 4 further wherein said incident beam of light isfocused in a manner such that it includes at least one ray that issubstantially normal to said surface of said sample.
 6. A method asrecited in claim 4 wherein said step of generating a beam of lightincludes using a laser light source.
 7. A method as recited in claim 4wherein said position sensitive detector is a photodiode detector.
 8. Amethod of measuring the characteristics of a thin film layer on thesurface of a sample comprising the steps of: generating a probe beam ofX-rays; focusing said probe beam on the surface of said sample such thatvarious X-rays within the focused probe beam create a range of angles ofincidence with respect to said surface; measuring the intensity ofvarious X-rays as a function of position within the probe beam asreflected with the positions of the X-rays within said reflected probebeam corresponding to specific angles of incidence with respect to saidsurface; and separately measuring the vertical position of said samplein order to either adjust said vertical position relative to the focusof said focused probe beam or to correct the X-ray measurements toaccount for the relative vertical position.
 9. A method as recited inclaim 8 wherein said relative vertical position of said sample isadjusted.
 10. A method of measuring the characteristics of a thin filmlayer on the surface of a sample including the steps of generating aprobe beam of X-rays; focusing said probe beam on the surface of saidsample such that various X-rays within the focused probe beam create arange of angles of incidence with respect to said surface; measuring theintensity of various X-rays as a function of position within the probebeam as reflected with the positions of the X-rays within said reflectedprobe beam corresponding to specific angles of incidence with respect tosaid surface; and separately measuring the vertical position of saidsample in order to either adjust said vertical position relative to thefocus of said focused probe beam or to correct the X-ray measurements toaccount for the relative vertical position, said vertical positionmeasuring comprising the steps of: generating a beam of light; passingsaid beam through a beam splitter; using a lens to focus said beam oflight on the surface of said sample; returning at least a portion of thereflected beam back through said lens to said beam splitter andreflecting it from said beam splitter; using a second lens to bring thebeam reflected from said beam splitter to a focus near a chopper;measuring the movement of the shadow cast by said chopper using aposition sensitive detector; and determining changes in the verticalposition of said sample based upon the measurements of said shadow. 11.A method as recited in claim 10 further wherein said beam of light isfocused in a manner such that it includes at least one ray that issubstantially normal to said surface of said sample.
 12. A method asrecited in claim 10 wherein said step of generating a beam of lightincludes using a laser light source.
 13. A method as recited in claim 10wherein said position sensitive detector is a photodiode detector. 14.An apparatus for measuring the characteristics of a thin film layer onthe surface of a sample comprising: means for generating a probe beam ofX-rays; means for focusing said probe beam on the surface of said samplesuch that various X-rays within the focused probe beam create a range ofangles of incidence with respect to said surface; means for measuringthe intensity of various X-rays as a function of position within theprobe beam as reflected with the positions of the X-rays within saidreflected probe beam corresponding to specific angles of incidence withrespect to said surface; and means for separately measuring the tilt ofsaid sample in order to either adjust said tilt or to correct the X-raymeasurements to account for said tilt.
 15. An apparatus for measuringthe characteristics of a thin film layer on the surface of a samplecomprising: means for generating a probe beam of X-rays; means forfocusing said probe beam on the surface of said sample such that variousX-rays within the focused probe beam create a range of angles ofincidence with respect to said surface; means for measuring theintensity of various X-rays as a function of position within the probebeam as reflected with the positions of the X-rays within said reflectedprobe beam corresponding to specific angles of incidence with respect tosaid surface; and means for separately measuring the vertical positionof said sample in order to either adjust said vertical position relativeto the focus of said focused probe beam or to correct the X-raymeasurements to account for the relative vertical position.
 16. A methodof measuring the characteristics of a thin film layer on the surface ofa sample including the steps of generating a probe beam of X-rays;focusing said probe beam on the surface of said sample such that variousX-rays within the focused probe beam create a range of angles ofincidence with respect to said surface; measuring the intensity ofvarious X-rays as a function of position within the probe beam asreflected with the positions of the X-rays within said reflected probebeam corresponding to specific angles of incidence with respect to saidsurface; and separately measuring the vertical position of said samplein order to either adjust said vertical position relative to the focusof said focused probe beam or to correct the X-ray measurements toaccount for the relative vertical position, said vertical positionmeasuring comprising the steps of: generating a beam of light; using alens to focus said beam of light on the surface of said sample;directing at least a portion of the reflected beam to a second lens;using said second lens to bring the reflected beam to a focus near achopper; measuring the movement of the shadow cast by said chopper usinga position sensitive detector; and determining changes in the verticalposition of said sample based upon the measurements of said shadow. 17.A method as recited in claim 16 further wherein said beam of light isfocused in a manner such that it includes at least one ray that issubstantially normal to said surface of said sample.
 18. A method asrecited in claim 16 wherein said step of generating a beam of lightincludes using a laser light source.
 19. A method as recited in claim 16wherein said position sensitive detector is a photodiode detector.